The conventional catalytic converter utilizes a ceramic monolith on which is deposited a catalyst which aids in the conversion of pollutants such as, carbon monoxide, nitrogen oxides (NO.sub.x), unburned hydrocarbons, etc., to carbon dioxide, nitrogen, and water. However, this conversion is not efficient initially while the exhaust gases are relatively cold. To be effective at a high conversion rate, the catalyst and surface with which the exhaust gases come in contact must be at a minimum elevated temperature; 390.degree. F. for carbon monoxide, 570.degree. F. for volatile organic compounds, and 1000.degree. F. for natural gas. Otherwise, the conversion to harmless by-products is low and cold-start pollution of the atmosphere is high. Of course, once the exhaust system comes to its operating temperature, the catalytic converter is effective. Hence, it is to the achievement of early conversion of pollutants before normal operating temperature is obtained or operation when low temperatures are encountered that the present invention is directed.
The principle of elevating the temperature of a catalytic converter to a higher more efficient operating temperature for start-up is not, per se, new. Reference may be had to U.S. Pat. No. 3,768,982 to Kitzner dated Oct. 30, 1973. In this patent, heat from a centrally located electric heater is transferred by conduction through a monolithic catalyst support to heat the catalyst to an optimum operating temperature. Reference may also be had to U.S. Pat. No. 3,770,389 to Kitzner dated Oct. 30, 1973 which discloses a central electrically heated core within a ceramic monolith, heat being transmitted by conduction to the catalyst contained in the openings of the ceramic monolith. The heating core is formed of metal sheets, one flat and one corrugated, coated with alumina and also bearing a catalyst. The metallic core is heated electrically by virtue of its own electrical resistance.
Recent attempts at electrically heating catalysts have shown that corderite ceramic and silicon carbide, while having considerable electrical resistance, are too brittle to accommodate the relatively large electrodes and high current flow required for electrically heated converters. Furthermore, earlier attempts at electrically heating the catalyst do not take advantage of the low backpressure associated with metallic honeycomb catalytic converters, nor do they take advantage of the very high heat transfer efficiency when the catalyst containing ferritic strip is also used as the electrical conductor and heating element.
Reference may be had to the U.S. Pat. No. 4,711,009 to Cornelison and Retallick, dated Dec. 8, 1987 for details of the preparation of accordion folded thin stainless steel honeycomb converter units having a catalyst supported on an aluminum oxide surface. This patent is incorporated herein by reference thereto.
The devices of the present invention are distinguished from the prior art structures in that they do not depend upon conduction to transmit heat, a time consuming event. In the present invention, the entire catalytic monolith is formed of corrugated thin metal foil which becomes the resistance heater and whereby the entire monolith is elevated in temperature rather than depending on conduction through a ceramic, for example, to achieve the desired temperature. The devices of the present invention will achieve optimum catalyst temperature in from 2 to 20 seconds depending upon the voltage potential, as compared to several minutes for converters depending on conduction as the mode of heat transfer. This is an important distinction if one is obligated to wait for a catalytic converter to achieve "light-off" or optimum operating temperature before starting the engine. Moreover, the corrugated metal foil structure is nonnesting and the necessity for a flat metal sheet is obviated. Still further, spirally wound devices as illustrated in the prior art are subject to telescoping of the catalyst support unit under operating conditions.